Air-cooled condensers, for example air-cooled steam condensers, are designed for full load operating capacity at relatively high ambient air temperature. During part of the time, however, such units may have to operate at low load and ambient air temperatures below the freezing temperature of the condensate, or at low ambient air temperatures. During such a period, the problem of freezing of condensate may arise. The possibilities and dangers of ice formation in the operation of air-cooled steam condensers have been the subject of numerous studies, investigations, patents and articles for quite some time. This subject has been of particular importance during the past 15 years or so, due to the increasing use of air-cooled heat exchangers for cooling and condensing steam or water vapor in chemical plants and refineries and for cooling water or condensing steam in large power plants. In the design of air-cooled steam condensers, therefore, it is necessary in many cases to provide means to prevent or at least lessen the possibility of damage to the tubes of the condensers resulting from ice formation.
As is well known, the danger of freezing of condensate in such equipment is of greatest concern in two situations: at higher than design heat transfer rates and at low steam flow rates. As those skilled in the art will realize, we are at all times herein concerned with ambient temperatures at or below the freezing point of water. The former situation may result from local prevalent weather conditions which engender low ambient air temperatures or unusually strong winds, or both; these may be encountered regularly and for long periods of time in some regions. The latter situation may occur in any installation because of factors such as partial shutdown of equipment or variations in the amount of steam produced.
Some approaches taken to deal with this problem have concentrated on designing an air-cooled steam condenser with structural features which would serve to control the condensation of steam so as to prevent or at least lessen the likelihood of ice formation. Thus, for example, it has been suggested to provide valves on condensate drains to prevent backing up of condensate which could conceivably cause ice formation, to provide separate manifolds for each row of condenser tubes to prevent backing up of condensate from one row of tubes into another row at a higher level, to provide different types of fins on the tubes of different rows of the steam condenser, or to provide some tubes with fins along only a portion of their length to equalize heat transfer, and to provide small tubes within the main tubes for removing condensate at a portion of the tubes between the steam inlet manifold and the condensate outlet manifold. Numerous other special designs have been proposed to achieve the aim of preventing freezing of condensate. Others have approached the problem from the aspect of controlling the air flow over the tubes and have provided fan controls of various types responsive to one or more conditions in the system to lessen the air flow over the tubes when the steam flow through the condenser is low, as otherwise too high a heat transfer rate would be effected between the large quantity of external cooling air and the small quantity of steam in the tubes, and ice formation within the tubes could result. Thus, systems have been proposed for shutting fans off, for lowering their speed, for changing the pitch of the fan blades, for covering the tube surfaces with movable baffles, and for otherwise controlling the amount of cooling air flowing over the tubes.
In situations in which low ambient temperatures or varying steam flow rates at below freezing temperatures are anticipated, it has become quite common to provide fan control systems (usually responsive to some thermal condition in the system) to either lower the speed of one or more fans or shut them down entirely until the normal flow of cooling air is again required. Thus, the lowering of fan speed or shutting down of fans descreases the quantity of cooling air flowing over the tubes of the condensers and will often prevent ice formation which could otherwise occur in such conditions due to an excessive heat transfer rate. However, even with fans shut off, there may still be an appreciable flow of cooling air over the tubes of the condensers due to either wind effects or natural convection currents, and thus the danger of freezing of condensate may still exist, especially when the flow of steam is quite below the normal rate.
In natural draft cooling towers, the admission of the cooling air is usually controlled by louvres or shutters, but air may still enter the cooling tower even when these are closed, and again there is the danger of freezing of condensate in this situation, when a low steam flow rate exists in one or more of the condenser units concurrently with air temperatures below the freezing temperature of the condensate.